1. Field of the Invention:
Our invention relates to optical signal processing in general and, more specifically to optical signal combiners for a plurality of optical signals over a broad frequency region.
2. Description of the Related Art:
Many optical signal processing applications require the combination or summing of signals. Positive-Intrinsic-Negative (PIN) photodiodes are frequently used in such applications as optical detectors because of their excellent spectral response and high speed. The PIN diode can be used to sum optical signals because it produces a photocurrent output that is directly proportional to radiant flux at the photosensitive base region when the spot size is vanishingly/respectively small. The ratio of photocurrent to spot flux is called the "responsivity" of the photodiode. Although responsivity is generally independent of base region area because it is defined for a vanishingly small spot size, responsivity varies greatly with distance from the center of the photodiode base region. Response speed of a PIN photodiode also varies with the distance of such a spot from the center. Accordingly, an extremely fast PIN photodiode designed to operate in the gigahertz (GHz) frequency region typically has an base region diameter on the order of 100 microns, with 40 micron diameters being not uncommon.
The PIN photodiode base region diameter is often large enough to couple the device to a single optical fiber, but the joint coupling of more than one such fiber to a PIN photodiode has not been achieved in the prior art. Instead, practitioners have been obliged to combine such signals using prior art devices that passively mix individual beams to form a single combined signal. The combined signal may then be transmitted to a photodiode by a single optical fiber that is coupled to the photodiode using one of a variety of methods known in the art. However, passive beam mixing and combining devices known in the art inherently cause losses of optical signal power that are unacceptable in many situations.
An integrated "combiner" device for passively combining optical signals is commonly used. This is a Y-shaped optical waveguide fabricated on a dielectric substrate that may also be used as a "splitter." Ideally, the angle at which the input branches converge is made infinitesimally small to virtually eliminate loss. Because actual devices cannot be fabricated with an infinitesimally small convergence angle, the actual convergence angle causes a 3 dB loss between inputs and the combined signal output. A tree composed of combiners has been used to combine large numbers of signals, but a tree of even moderate size creates a significant signal-to-noise ratio (SNR) problem because each node contributes a 3 dB loss. For example, combining eight signals requires a three-level tree, resulting in a 9 dB signal loss at the combined output.
Another related integrated device is the "star coupler," which is a passive device having a plurality of optical waveguide inputs that converge to a common "mixing region," thereby combining the input signals, and a plurality of waveguide outputs that fan out from the mixing region. The N.times.N star coupler is useful for applications where N input signals must be combined and routed to N outputs, as is commonly required in communications networks. However, to evenly distribute the combined signal over all N output waveguides, the width of the mixing region must be significantly larger than that of the individual waveguides. A star coupler cannot be efficiently used as a combiner because each output waveguide receives only one-Nth the combined signal power. Assuming that a star-coupler could be split in half at the mixing region to yield a coupler having a single output, avoiding the factor-of-N power reduction, the mixing region end of the resulting device would still be too large for direct coupling to a PIN photodiode.
Star couplers have been constructed from optical fibers using the "biconical taper" and "fused head-end" techniques, which are described at length by P. J. Severin, et al., "Passive Components for Multimode Fiber-Optic Networks", Journal of Lightwave Technology, Vol. 4, No.5, pp. 491-492, May, 1986. The biconical taper technique involves fusing the tapered midsections of two fibers together at high temperatures. Large star couplers must be formed by similarly fusing smaller ones together.
The technique does not work well for large star couplers because signal mixing is often uneven. The fused head-end (FHE) technique described by Severin, et al. involves removing the cladding from the first centimeter at the end of each input fiber, thereby exposing the core, and packing the exposed cores together as tightly as possible in a symmetrical pattern within a capillary tube. Under carefully controlled conditions of temperature and pressure, the capillary tube shrinks and the fibers deform to fill the interstices. The capillary tube then serves as "cladding" over the deformed etched portion of the fibers. The FHE is then drawn to further reduce the diameter of the deformed "core" to approximate that of a single fiber core. However, a 3 dB loss is introduced between inputs and the combined signal output as a result of the drawing process. The resulting FHE is typically coupled to a single fiber to form a splitter or combiner, adding additional signal loss at the discontinuity between the fiber and the FHE.
The FHE technique has generally been limited to multimode fibers because of their larger core diameters. However, an increasing number of commercially available optical devices are using single-mode fibers, which cannot be efficiently coupled to multimode fibers. FHE fabrication using single-mode fibers is very difficult because single-mode fibers are extremely fragile when etched down to their small cores. Single-mode star couplers are known in the art but such star couplers of even moderate size are extremely fragile, requiring complex and expensive packaging techniques such as suspending the coupler joint in a refractive potting compound.
There is a strongly-felt need in the art for an economical, low loss optical power summing device that uses single-mode fibers. These unresolved problems and deficiencies are clearly felt in the art and are solved by our invention in the manner described below.